The present invention relates to transparent yttria ceramics and to methods for the preparation thereof. More particularly, this invention relates to substantially transparent, high density, polycrystalline yttria doped with magnesium oxide or magnesium aluminate and to methods for the preparation thereof.
Ceramics have long been considered desirable materials for use in high temperature applications; however, ceramic materials are generally opaque and cannot be suitably employed where light transmission is desired. Efforts have heretofore been made to obtain transparent ceramics especially for high temperature lamp envelope applications.
For example, U.S. Pat. No. 3,545,987 to Anderson discloses a high-density, yttria-based polycrystalline ceramic containing from about 2 to 15 mole % of an oxide selected from the group consisting of thoria, zirconia, hafnia or combinations thereof. It has been found, however, that in such materials anion interstitial point defects are formed which are unstable in the low-oxygen pressure, high temperature environment of a high-pressure sodium vapor lamp. Consequently, high-pressure sodium lamps having envelopes formed from such materials have been found to be unsatisfactory since they darken rendering the envelope opaque after a few hours of use.
U.S. Pat. No. 3,878,280 describes a method for the preparation of transparent yttria without the use of additives by the use of a vacuum hot pressing operation. This technique is not amenable, however, to the production of thin-wall lamp envelope tubing due to high-pressure gradients along the length of the tube. Consequently, hot pressing is not currently considered a viable process for the formation of transparent polycrystalline tubes.
U.S. Pat. No. 3,873,657 describes the preparation of transparent yttria by employing beryllium compounds as sintering aids. The beryllium compounds employed, however, are toxic; consequently, utilization of such materials in commercial manufacturing operations would be severely handicapped.